At the present time, the process of setting up and conducting a typical large 3D seismic survey in land or transition zone environment involves laying out an array of substantially parallel main seismic data cables, each typically several thousand meters long and each having a large number of sensor strings connected to them at respective connection nodes, over the area to be surveyed. The main cables are typically spaced about 400 meters apart, while the connection nodes are typically spaced at about 50 meter intervals along the main cables. Each sensor string typically has as many as 10 analogue or digital geophones or equivalent seismic sensors connected in it, the sensors of a single string forming a group whose outputs are combined (in the simplest case, summed), and the centres of the groups attached to a given main cable being substantially aligned along a line coincident with or substantially parallel to that main cable.
Once the array of main cables and seismic sensors is laid out as described above, at least one seismic source, typically a vehicle-mounted vibrator, traverses the survey area, along lines substantially perpendicular to the main cables and also typically 400 meters apart. The vehicle typically stops every 50 meters, and the vibrator is energised to direct a swept frequency acoustic signal downwardly into the earth formations beneath the survey area, for reflection by the formations and detection by the sensors. As the survey progresses, the main cables at one end of the array may be successively transferred to the other end, to effectively move the array along the survey area in the direction in which the source or sources move.
It will be appreciated that the above described process is very expensive, inter alia because it requires a large number of field personnel and takes a substantial amount of time. It is an object of the present invention to provide methods and apparatus which make the process more efficient.
According to a first aspect of the present invention, there is provided apparatus for use in seismic data acquisition in a land or transition zone environment, the apparatus comprising:
at least one seismic data cable which, in use, is laid out along a line of the area to be seismically surveyed, said cable having a plurality of spaced apart connection nodes therealong; and
a plurality of sensor strings connected to said connection nodes, each sensor string comprising a plurality of seismic sensors each adapted to produce a respective output signal representative of a seismic signal received thereby;
wherein the sensor strings are adapted to permit the sensors to be laid out in at least two sets of groups, each group containing a plurality of the sensors and being adapted to transmit at least one seismic signal to a connection node, and the sets of groups being disposed along respective spaced apart lines generally parallel to the line along which the seismic data cable is laid.
In a preferred embodiment of this first aspect of the invention, each sensor is adapted to produce a digital output signal representative of the seismic signal received thereby, and a respective digital identifying signal which identifies the sensor, and is connected to transmit both digital signals to the respective connection node.
The sensors of each sensor string may be divided between a group in the first set and a group in the second set (although this is not strictly necessary), in which case the sensor string preferably includes a portion of increased length between the sensors in the group in the first set and the sensors in the group in the second set, to facilitate the spacing apart of the groups.
According to a second aspect of the invention, there is provided a method of seismic data acquisition, the method comprising:
laying a seismic data cable along a line of the area to be seismically surveyed, said cable having a plurality of spaced apart connection nodes therealong, and a plurality of sensor strings connected to said connection nodes, each sensor string comprising a plurality of seismic sensors each adapted to produce a respective output signal representative of a seismic signal received thereby;
laying the sensors of the sensor strings out in at least two sets of groups, each group containing a plurality of the sensors and transmitting at least one seismic signal to a connection node, and the sest of groups being disposed along respective spaced apart lines generally parallel to the line along which the seismic data cable is laid;
emitting seismic signals downwardly into the area to be surveyed, for reflection by earth formations beneath the area; and
detecting the reflected signals with the sensors.
In a preferred implementation of this second aspect of the invention, each sensor is adapted to produce a digital output signal representative of the seismic signal received thereby, and a respective digital identifying signal which identifies the sensor, and both digital signals are transmitted to the respective connection node.
In practice, the at least one-seismic data cable is successively laid, or more conveniently a plurality of such seismic data cables are concurrently laid, along generally parallel lines, which are typically several hundred meters apart, while the respective lines along which the sets of sensors are disposed are typically less than 100 meters, eg 50 meters, apart.
The sensors of each sensor string may be divided between a group in the first set and a group in the second set (although this is not strictly necessary), in which case the method preferably further comprises including a portion of increased length in each sensor string between the sensors in the group in the first set and the sensors in the group in the second set, to facilitate the spacing apart of the groups.
Conveniently, the acoustic signals are emitted at substantially uniformly spaced points along at least one line substantially perpendicular to the seismic data cable, and the spacing of the respective lines of the sets of groups of sensors connected to the seismic data cable is substantially equal either to the spacing of said points, or to half the spacing of said points.
The sensor strings may be connected to the seismic data cable either before or after the seismic data cable is laid.